When you symlink a directory, you are telling nginx that "when you need to use /var/www/html/phpmyadmin, you should instead look at /usr/share/phpmyadmin/. And that directory is not under your webroot directory, so nginx won't be using it.
Instead of using a symlink, tell nginx to start using that directory directly. Example:
location /phpmyadmin { index index.php index.html index.htm; root /usr/share; } That will tell nginx that the location /phpmyadmin lives under /usr/share instead of under /var/www/html/.
Or, if the /usr/ and the /var/ file systems are on the same partition, you could do a hard link instead of a symlink. But that's likely to cause problems for you if you ever change the partition layout.
root /usr/share must be root/usr/share;